A megalomaniac is a self-centered, power-crazed egomaniac that is obsessed with their goals for conquering cities and countries and world domination, they're rather over-obsessed with power rather than actual conquest. They also often fantasize of greatness, fame, wealth, grandeur, omnipotence predominate and servitude. Because of all the power driving them to becoming critically insane their minds are separated from reality since they also don't care what happens to anything at all as they care about themselves.
Despite having the word maniac in the name of them being a megalomaniac, it does not mean they are always clinically insane.
People in the second group have a slight case of megalomania. And yet they are obsessed with world domination as on their record they was labeled a megalomaniac